Lifting magnet for sheet-formed objects of magnetisable material

ABSTRACT

LIFTING MAGNETS HAVING AN INTERMEDIATE POLE OF MAGNETIZABLE MATERIAL WHICH IS INSERTED BETWEEN TWO COAXIAL MAGNETIC COILS. EACH OF THE COILS HAS SUBSTANTIALLY THE IDENTICAL NUMBER OF AMPERE TURNS. THE LIFTING MAGNET HAS A RELATIVELY HIGH CARRYING CAPACITY WHEN THE TWO COILS ARE FED WITH CURRENT WHICH PERMITS THE MAGNET TO LIFT A SINGLE THIN SHEET FROM A STACK OF SHEETS OF MAGNETIZABLE MATERIAL.

Jan. 12, 1971 J. MARCHER LIFTING MAGNET FOR SHEET-FORMED OBJECTS OFMAGNETISABLE MATERIAL 2 Sheets-Sheet 1 Filed April 29, 1969 3/ 8 \2 TM 09 f INVENTOR.

JdRGi/V MA Renal ATT RNEYS Jan. 12, 1971 J. MARCHER 3,555,474

v LIFTING MAGNET FOR SHEET-FORMED OBJECTS OF MAGNETISABLE MATERIAL FiledApril 29, 1969 2 Sheets-Sheet 2 fl m 5) AH/79.4) 1 I I I I v I 35 8/072/5 2'0 25 mm INVIiN'IOR.

BY JflRGEN "ARCHER Arron NEYS United States Patent 3,555,474 LIFTINGMAGNET FOR SHEET-FORMED OBJECTS OF MAGNETISABLE MATERIAL Jorgen Marcher,Soborg, Denmark, assignor to H. Nielsen & Son Maskinfabrik A/S,Aldersrogade, Copenhagen N., Denmark Filed Apr. 29, 1969, Ser. No.820,172 Claims priority, application Great Britain, May 14, 1968,22,934/ 68 Int. Cl. H01f 7/20 US. Cl. 335289 2 Claims ABSTRACT OF THEDISCLOSURE Lifting magnets having an intermediate pole of magnetizablematerial which is inserted between two coaxial magnetic coils. Each ofthe coils has substantially the identical number of ampere turns. Thelifting magnet has a relatively high carrying capacity when the twocoils are fed with current which permits the magnet to lift a singlethin sheet from a stack of sheets of magnetizable material.

The invention relates to a lifting magnet for sheetformed objects ofmagnetisable material and with at least one magnet coil which may beconnected to a source of current and which is carried coaxially in apreferably circular cylindrical yoke having a flat underside Withannular pole surfaces.

Lifting magnets of the aforesaid kind are known and they are used amongother things for transporting sheets of magnetisable material such asmild steel by means of a magent crane in which the lifting magnet issuspended. In that case it is advantageous if it is possible totransport partly a stack of such sheets, partly to lift a single thinsheet from such a stack. In the known lifting magnets the latter maytake place, for example by first lifting a number of sheets from thestack, subsequently cutting off current to the magnet coil for briefintervals one or more times, as a consequence of which the lowermostsheets will be separated off, leaving ultimately only a single sheetadhering to the lifting magnet.

This known method of separating the sheets has, however, certaindrawbacks; among other things, it is time consuming, and the sheets maybe damaged when falling. Further, a relatively extensive electronicequipment is required in automatic cranes to ensure satisfactoryseparation of the uppermost sheet from the remaining sheets of thestack, and in adjusting and operating this equipment special allowancehas to be made for the thickness of the sheets.

Another known method consists in gradually reducing the terminal voltageto the magnet coils, by which a gradual reduction of the carryingcapacity of the lifting magnet is obtained until it lifts a single sheetonly. However, a substantial reduction of the terminal voltage to themagnet coils is required with consequent reduction of the resultingmagnetic field in order to obtain a substantial reduction in thecarrying capacity, and such a reduction in the terminal voltage suppliedto the magnet coils involves a considerable factor of uncertainty, sincethe lines of force of the weakened field are distributed over the entirepole surface area and since the carrying capacity of the lifting magnetis inversely proportional with the pole surface area.

It is the object of the invention to devise a lifting magnet of theaforesaid kind having none of the drawbacks referred to, and anessential feature of a lifting magnet according to the invention is thatan intermediate pole of magnetisable material which is common for themagnetic circuit of both of two coaxial magnet coils is insertedPatented Jan. 12, 1971 between the said two coils, both of which are sodimensioned that, when connected to the source of current, each of themseparately has substantially the identical number of ampere turns.

As a result, the lifting magnet will have a relatively high carryingcapacity when the two coils are fed with current so that they producemagnetic fields with the same radial direction in the sheets adhering tothe pole surface, the said fields being added to the resulting fieldwhich is of great strength and has a great capacity of penetrating intothe sheets and extends outside the intermediate pole, but through theouter and inner pole of the magnet. If the direction of the currentsupplied to one coil is reversed, the two coils will produce magneticfields of radially opposed directions in the sheets, and these fieldswill be subtracted to a resultant field having a lower strength and lesscapacity of penetrating into the sheets, the said field extendingconcentrated through the pole surface area of the intermediate pole andthrough the pole surfaces of the outer and the inner poles. Thedifference of the lines of force of the magnet fields in the two casesmay be utilised, partly for carrying a whole stack of sheets ofmagnetisable material, partly for lifting only the uppermost thin sheetfrom the remaining sheets of the stack.

In one embodiment of the lifting magnet according to the invention thepole surface of the intermediate pole is of an area which is equal toone fourth of the area of each of the pole surfaces in the magneticcircuit which surrounds both the coils.

The invention will now be further described by way of example and withreference to the drawing, in which FIG. 1 shows a diametrical section ofan embodiment of a lifting magnet according to the invention and with anintermediate pole the pole surface area of which is equal to one fourthof the pole surface areas of the magnetic circuit which surrounds bothmagnet coils of the lifting magnet,

FIGS. 25 show diagrammatically magnetic circuits in same in fourdifferent combinations of the current supply to the magnet coils of thelifting magnet,

FIG. 6 shows curves of the carrying capacity W of the same liftingmagnet as a function of the sheet thickness T when the magnet coils ofthe lifting magnet are fed with current as indicated in FIGS. -4 and 5,respectively, and

FIG. 7 shows curves for a force S as further described below, the saidforce being referred to as the separating force, at varying terminalvoltage for the lifting magnet according to the invention, partly incases where the lines of force of the magnetic fields are as indicatedin FIG. 4, partly in cases where the lines of force of the magneticfields are as indicated in FIG. 5.

The lifting magnet according to the invention as illustrated in FIG. 1has two coaxial magnet coils, an inner coil 1 and an outer coil 2, eachof which may be connected to a source of direct current (not shown) andcarried in 2. preferably circular cylindrical yoke 3 of magnetisablematerial. The said yoke has a flat underside 4 with annular polesurfaces for an inner pole 5 and an outer pole 6. The lifting magnet hasfurthermore suspension means (not shown) connecting it to a hoistingmechanism or the lifting means of a magnet crane. According to theinvention an intermediate pole 7 of magnetisable material is insertedbetween the two axial magnet coils 1 and 2, the said pole 7 being commonfor the magnetic circuits X and Y, respectively, of both coils and itsmagnetic cross-section Q being at right angles to the magnetic lines offield in the embodiment illustrated in FIGS. l-5 substantially smallerthan the cross-sections Q, and Q of the magnetic circuit surroundingboth of the coils 1 and 2. The magnet coils 1 and 2 are so dimensionedthat, when connected to the current source (not shown), each of themhave substantially the identical number of ampere turns.

When a lifting magnet according to the invention and of the type shownin FIG. 1 is to be used to lift either a whole stack of sheets ofmagnetisable material or only the uppermost thin sheet of the stack, thedepth of penetration of the magnetic fields into the object to be liftedhas to be varied, and the said penetration depends upon the followingthree factors:

(1) The magnetic resistance of the object to be lifted,

(2) The flux produced by the magnet coils,

(3) The area and shape of the pole surfaces.

The factor 1 will be fixed for each available object and cannot bealtered.

The factor 2 can be varied within the range of zero to maximum byvariation of the number of ampere turns of the magnet coils.

The factor 3 is fixed best possible by dimensioning the lifting magnetso as to obtain a maximum carrying capacity W in connection with a wholestack of sheets and an optimum separating force S, that is, the minimumforce to be applied on a central pull to separate a thick sheet,corresponding to a stack'of sheets, from an uppermost sheet lying indirect contact with the underside of the lifting magnet. A separatingforce is measured by means of a dynamometer, and the Whole forceincludes the weight of the thick sheet.

The distribution of the magnetic flux in a lifting magnet as indicatedin FIG. 1 is substantially determined by the size of the air gap betweenthe underside of the magnet and the object, and if there is theidentical size of the gap at all of the three pole surfaces when anobject is resting in contact with the underside of the magnet, themagnetic resistances in the three air gaps at the outer pole, theintermediate pole and the inner pole will be approximately proportionalwith the area of the corresponding pole surface.

In one embodiment of the lifting magnet according to the invention thepole surface of the intermediate pole has an area equal to one-fourth ofthe pole surface areas of the magnetic circuit surrounding both magnetcoils, by which a satisfactory proportion in practice between thecarrying capacity W of the lifting magnet is obtained when the magnetcoils 1 and 2 produce magnetic fields as indicated in FIG. 5 and thewhole separating force S oc curring when the two coils produce magneticfields as indicated in FIG. 4.

FIGS. 2-5 show the distribution of field lines at various combinationsof the current supply to the two magnet coils 1 and 2 provided, asindicated above, that the pole surface area Q of the intermediate poleis equal to onefourth of the pole surface areas of the outer and innerpole, Q and Q respectively.

With the directions of current, indicated by normal symbols in FIGS.2-5, the pole surfaces will have the north and south poles indicated byN and S, the numerical values representing the relative fluxdistribution between the pole surfaces and a sheet-formed object 8resting in contact with these, and it is a further prerequisite that theobject is not magnetised beyond the limit of magnetic saturation of thematerial.

FIG. 2 shows poles and relative flux distribution when only the innercoil 1 receives current and produces a field X with a relative fluxvalue of 5 in the sheet-formed object 8.

FIG. 3 shows poles and relative flux distribution when only the outercoil 2 receives current and produces a field Y, likewise with a relativeflux value of 5, in the sheetformed object 8.

FIG. 4 shows poles and relative flux distribution when both the innercoil 1 and the outer coil 2 receive current so that the fields produced,X and Y, produce a relatively weak field XY with a relative flux valueof 1 in the sheet-formed object 8.

FIG. 5 shows poles and relative distribution of fiux when both the innercoil 1 and the outer coil 2 receive current so that the resulting fieldsX" and Y" as a result produce a relatively strong field X"+Y" with arelative flux value 9 in the sheet-formed object 8.

On comparing FIGS. 4 and 5, it will be seen that in the first case theresulting field in the intermediate pole 7 has a value of two relativeunits, and that the field X'+Y extends through the intermediate pole 7,which has a south pole at its pole surface, whereas the inner pole andthe outer pole has a north pole at their pole surfaces.

In the second case (FIG. 5) the resulting field is of nine relativeunits in the outer and inner poles, and it extends between the outerpole 6, which is a north pole, and the inner pole 5, which is a southpole, whereas the two fields XY" anyway approximately will eliminateeach other in the intermediate pole 7.

The carrying capacity W of the lifting magnet may, as is known, becalculated with substantial approximation out from the formula:

2 2 constant kg.

(a) If this formula is applied to the case of FIG. 4, the result arrivedat will be a total relative carrying capacity W=1 /1+2 /%+1 l=18relative units of force.

(b) If the intermediate pole 7 is omitted and the terminal voltage ofthe two magnet coils is reduced so that the flux values consititute arelative unit both in the outer pole 6 and in the inner pole 5, we have:W=1 /1+1 /l=2 relative units of force, which shows that for the samerelative magnetic field in the sheet 8 to be lifted a ninefold carryingcapacity of the lifting magnet will be obtained by providing the liftingmagnet according to the invention with an intermediate pole 7.

(c) If the formula is applied to the case according to FIG. 5, therelative carrying capacity obtained in the magnet will be: W: 9 l+9 1:162 relative units of force, which means a ninefold carrying capacity ofthe lifting magnet in the case illustrated in FIG. 5 compared with thecase illustrated in FIG. 4.

(d) If the intermediate pole is omitted in the case of FIG. 5, this willnot have any appreciable influence upon the carrying capacity W of thelifting magnet, which has been confirmed by tests, and this is due tothe fact that the fields of the intermediate pole 7 eliminate each otheras indicated in FIG. 5.

The conclusion derived thereby is that by reversing the current ofmagnetisation in one of the magnet coils 1 or 2 of the lifting magnetaccording to the invention, that is, from the case illustrated in FIG. 5to the case illustrated in FIG. 4, the carrying capacity W of thelifting magnet will be changed from 162 to 18 relative units of force.

In the case illustrated in FIG. 4 the relative flux value in thesheet-formed object 8 will be a relative unit, and this means that thefields are so relatively small that their lines of force extendessentially in the uppermost sheet which is retained to the liftingmagnet by eighteen relative units of force, calculated under (a), andthe fields do not penetrate to any appreciable depth into the underlyingsheet.

If the intermediate pole 7 is omitted, that is, as in the known liftingmagnets, a regulated reduction of the terminal voltage of the liftingmagnet until the relative flux value of outer poles and inner pole isequal to one relative unit, the uppermost sheet of a stack will only beretained by two relative units of force as indicated under (b), that is,by only one ninth of the force produced in the lifting magnet accordingto the invention, there is thus a considerably greater risk of theuppermost sheet of the stack not being retained, but, on the contrary,the said sheet will fall simultaneously with the remaining part of thestack.

The above theoretical calculations have been confirmed by tests, theresults of which are indicated in the graphs 6 and 7.

FIG. 6 shows curves of values measured of the carrying capacity W of aconvenient embodiment of the lifting magnet described above by way ofexample as used in connection with various sheet thicknesses T, and itmay be added that a lifting magnet of the same type, but withoutintermediate pole has under the same conditions and with the magnetcoils 1 and 2 fed with current as indicated in FIG. 5 a maximum carryingcapacity of about 2500 kg. at a sheet thickness of 50 mm., the carryingcapacity varying with the sheet thickness as indicated by a curve A.

Another curve B shows that the lifting magnet according to the inventionhas a maximum carrying capacity about less, that is about 2200 kg. at asheet thickness of 50 mm., than a lifting magnet without intermediatepole (Curve A). The lifting magnet according to the invention has asshown by a curve C a carrying capacity of about 200 kg. when the magnetcoils 1 and 2 are fed with current as indicated in FIG. 4. The curve Cfurther shows that the carrying capacity in the case of FIG. 4 is ratherindependent of the sheet thickness.

FIG. 7 shows curves measured for the separating force S of a liftingmagnet according to the invention at a constant direct voltage of 110volts as a function of the thickness of the uppermost sheet. A curve Dshows that at the said direct voltage and a magnetisation of the liftingmagnet corresponding to FIG. 5 the separating force S will be below 50kg. when the sheet thickness T is greater than mm. If, however, thesheet thickness if less than 15 mm. and the magnetisation is asindicated in FIG. 4, a substantially larger separating force isrequired, that is, as far as up to 550 kg. and 850 kg. in the case ofsheet thicknesses T, 5 and 3 mm., respectively. Whether in such casemore than a single sheet will be lifted, will thus depend on the lengthand width of the sheet, since the sheet lying beneath the first one willbe carried with if its weight is less than 550 kg. and 850 kg.,respectively. By means of a lifting magnet according to the inventionthe separating force will, however, as shown by a curve E, with the twomagnet coils connected as indicated in FIG. 4 be very low, since 6 evenin the case of a 3 mm. sheet the separating force required will only be30 kg., that is, a 3 mm. sheet of about 1.3 m. may be separated fromanother sheet of any thickness whatever or from a stack of any size whatever consisting of 3 mm. sheets.

Corresponding tests with a lifting magnet without intermediate pole,that is, like the known lifting magnets, have disclosed that the voltageto the magnet coils has to be reduced to substantially less than 20volts in order to obtain a separating force S in the case of a 3 mm.sheet for less than 100 kg., but simultaneously the carrying capacity Wwill only be about kg. at 20 volts, and the known lifting magnet willthus drop the stack of sheets before the uppermost 3 mm. sheet isseparated from the stack.

What we claim is:

-1. A lifting magnet for sheet-formed objects of magnetizable materialand with at least one magnet coil which may be connected to a source ofcurrent and which is carried coaxially in a preferably circularcylindrical yoke having a flat underside with annular pole surfaces,characterized in that an intermediate pole of magnetizable materialwhich is common for the magnetic circuit of both of two coaxial magnetcoils is inserted between the said two coils, both of which are sodimensioned that, when connected to the source of current, each of themseparately has substantially the identical number of ampere turns.

2. A lifting magnet as claimed in claim 1, characterized in that thepole surface of the intermediate pole is of an area which is equal toone fourth of the area of each of the pole surfaces in the magneticcircuit which surrounds both the magnet coils.

References Cited UNITED STATES PATENTS 3,409,806 11/1968 Fritz 335-289XFOREIGN PATENTS 1,016,997 l/l966 Great Britain 335289 GEORGE HARRIS,Primary Examiner U.S. Cl. X.R.

